1. Field of the Invention
The present invention relates to a technique that in a liquid ejection apparatus having a plurality of chips arranged in a specific direction, each chip having a plurality of liquid ejection units juxtaposed in the specific direction, the displacement of a liquid ejection direction between the chips is reduced.
2. Description of the Related Art
Inkjet printers have been known as an example of a liquid ejection apparatus having a plurality of chips juxtaposed in a specific direction. As ink ejecting systems of the inkjet printer, there are a thermal system for ejecting ink using thermal energy and a piezoelectric system for ejecting ink using a piezoelectric element.
Also, from a viewpoint of an ink color, there are a single color type using one printer-head chip and a color type using a plurality of printer-head chips while ink with different color is ejected from each chip.
Furthermore, from a viewpoint of a head structure, there are a serial system using one printer-head chip for each color, which is moved in the width direction of printing paper for printing images thereon, and a line system having a number of printer-head chips juxtaposed in the width direction of printing paper for each color so as to form a line head for a width of the printing paper.
FIG. 12 is a plan view of a line head 10. In FIG. 12, four printer-head chips 1 (“N−1”, “N”, “N+1”, and “N+2”) are shown; however, further more numerous printer-head chips 1 are arranged in practice.
Each printer-head chip 1 has a plurality of nozzles la formed therein, each having an ejection hole for ejecting ink. The nozzles la are juxtaposed in a specific direction, which agrees with the width direction of printing paper. Furthermore, a plurality of the printer-head chips 1 are arranged in the above-mentioned specific direction. Adjacent printer-head chips 1 are arranged so that the respective nozzles la face each other while pitches of the nozzles la between adjacent printer-head chips 1 are in consecutive order (see a portion A in detail).
Moreover, an example of a structure of the above-mentioned thermal system printer-head chip has been known, which has an ink liquid chamber and a heating resistor arranged in the ink liquid chamber so as to pressurize (heat) ink ejecting within the ink liquid chamber. The nozzle is formed on the upper surface of the ink liquid chamber and constituted such that the ink pressurized within the ink liquid chamber is ejected from the ejection hole of the nozzle.
In addition to the example having a single heating resistor within the ink liquid chamber, another one having a plurality of heating resistors divided within one ink liquid chamber has been known.
FIG. 13 is a plan view of an example having two-divided heating resistors within one ink liquid chamber. The region of the ink liquid chamber 2 is substantially circular, and a flow path 2a communicated with an ink liquid chamber 2 is formed in the lower part of the drawing. Furthermore, two heating resistors 3 are arranged within the ink liquid chamber 2 in the lining-up direction of nozzles (in the right and left viewing the drawing).
As such an example, in a divided type in which the heating resistor 3 is halved longitudinally, since the width is halved while the length is the same, the resistance of the heating resistor 3 is doubled. If the two-divided heating resistors 3 are connected in series, the resistance quadruples.
The reason of such a structure is as follows.
In order to film-boil ink (a phenomenon of an entire surface boiling membranously) in the ink liquid chamber 2; it is necessary to heat the heating resistor 3 by supplying a predetermined amount of electric power to the heating resistor 3. By means of energy during the film boiling, the ink is ejected. If the resistance is small, it is necessary to increase the electric current to be passed, so that by increasing the resistance of the heating resistor 3, the film boiling can be performed with the small current.
Thereby, a transistor for passing electric current can be reduced in size, enabling a space to be reduced. In addition, the reduction in thickness of the heating resistor 3 increases the resistance; however, in view of the material and the strength (durability) selected as for the heating resistor 3, there is a predetermined limit in the reduction of thickness. Therefore, the resistance has been increased by dividing the heating resistor 3.
However, there have been the following problems in the conventional technique described above.
(First Problem)
First, during the ejecting ink from the printer-head chip, it is ideal that the ink be ejected perpendicularly to the ejection surface of the printer-head chip; however, there are cases where the ejecting angle of ink is not perpendicular depending on various factors.
For example, in the thermal printer-head chip, during the bonding a nozzle sheet having nozzles formed thereon, on the upper surface of the ink liquid chamber having the heating resistors 3, the displacement in the bonding position between the ink liquid chamber/the heating resistors 3 and the nozzles arises a problem. If the nozzle sheet is bonded so that the center of the nozzle is located at the center of the ink liquid chamber/the heating resistors 3, ink is ejected perpendicularly to the ink ejection surface (the nozzle sheet surface); however, if the center positions of the ink liquid chamber/the heating resistors 3 and the nozzles are displaced, the ink is not ejected perpendicularly to the ejection surface.
Also, the displacement due to the difference in the thermal expansion coefficient between the ink liquid chamber/the heating resistors and the nozzle sheet may be produced.
When ink is ejected perpendicularly to the ejection surface, an ink drop is landed at a precise position. If the ejecting ink is displaced by an angle of θ from perpendicularity, the displacement of the landing position of the ink drop ΔL is expressed by:ΔL=G×tan θ, wherein the distance between the ejection surface and the printing paper surface (the landing surface of the ink drop) is G (generally 1 to 2 mm for the inkjet system).
When such displacement in the ink ejection angle is generated, the image quality is not noticed so much in the serial system, whereas in the line system, it becomes a problem. This will be described as follows.
FIG. 14 includes a sectional view and a plan view showing an image-printing state using a head 1A having one printer-head chip in the serial system. In the sectional view of FIG. 14, if printing paper P is considered as being fixed, the head 1A moves so as to print images on the printing paper P in the conveying direction of the printing paper P in the drawing while reciprocating in the width directions of the printing paper P. The sectional view of FIG. 14 shows passing positions of the head 1A for the N-th and the (N+1)-th time.
Also, as shown by an arrow in the sectional view, FIG. 14 shows an example of the ink ejected at a slant in the left in the drawing, i.e., in the conveying direction of the printing paper P. At this time, the ink landing position is displaced in the left in the drawing; however, even if the ink is ejected at a slant at the N-th moving of the head 1, for example, the ink is ejected at the same angle also at the (N+1)-th moving. Therefore, the connection portion between the landing position of the head 1A in the movement for the N-th time and the landing position of the head 1A in the movement for the (N+1)-th time is not noticeable. That is, the reason is that the image printing is performed by the same head 1A having the same ejection characteristics both at the N-th and the (N+1)-th moving.
Also, in the case where ink is ejected at a slant in the moving direction of the head 1A, although the ink is landed out of alignment with the reference position at both ends in the width direction of the printing paper P, the ink landing position at the ends in the width direction of the printing paper P is not changed between passing positions of the head 1A for the N-th and the (N+1)-th time. Therefore, also in this case, the displacement of the ink landing position is not noticeable.
In the case where a plurality of color printer-head chips are provided, ink ejection characteristics may be different for each printer-head chip, and color misalignment is produced in this case. However, since the resolution of the color misalignment in human eyes is not so large, the misalignment of a single color is scarcely recognizable even in the case of documents where the color misalignment is mostly recognizable.
For color images such as a photograph, a technique may be frequently used, in which ink is landed in plural times with different nozzles in one printer-head chip for the same color so as to defuse the displacement within the printer-head chip, so that the color misalignment is scarcely recognizable.
FIG. 15 includes a sectional view and a plan view showing an image-printing state in the line head 10 shown in FIG. 12 (line head having a plurality of printer-head chips 1 arranged in the lining-up direction of the nozzles 1a). Referring to FIG. 15, if printing paper P is considered as being fixed, the line head 10 does not move in the width direction of the printing paper P but moves from the upper to the lower direction in the plan view so as to print images.
In the sectional view of FIG. 15, three N-th, (N+1)-th, and (N+2)-th printer-head chips 1 of the line head 10 are shown.
The sectional view shows examples that in the N-th printer-head chip 1, ink is ejected at a slant in the left of the drawing as shown by an arrow; in the (N+1)-th printer-head chip 1, ink is ejected at a slant in the right of the drawing as shown by an arrow; and in the (N+2)-th printer-head chip 1, ink is ejected vertically without slanting as shown by an arrow.
Accordingly, in the N-th printer-head chip 1, ink is landed away from the reference position in the left; and in the (N+1)-th printer-head chip 1, ink is landed away from the reference position in the right. Therefore, between both the printer-head chips 1, ink is landed in the direction moving away from each other. As a result, between the N-th printer-head chip 1 and the (N+1)-th printer-head chip 1, a region is formed where ink is not ejected. The line head 10 does not move in the width direction of the printing paper P but only moves in an arrow direction in the plan view. Thereby, between the N-th printer-head chip 1 and the (N+1)-th printer-head chip 1, a white stripe B is produced, reducing printed image quality.
Also, in the same way as mentioned above, since in the (N+1)-th printer-head chip 1, ink is landed at a slant away from the reference position in the right, between the (N+1)-th printer-head chip 1 and the (N+2)-th printer-head chip 1, a region is formed where landing positions of the ink are overlapped with each other. Thereby, there has been a problem in that images are discontinuous or stripes C are produced so as to reduce printing image quality.
Other than the case where the ink landing position of each printer-head chip 1 is displaced in the lining up direction of the nozzles as described above, it may be displaced in the moving direction of the printing paper P in some cases. FIG. 16, in the same manner as in FIG. 15, includes a sectional view and a plan view showing a printing state in the line head 10.
FIG. 16 shows an example in that the ink landing positions of the N-th printer-head chip 1 and the (N+2)-th printer-head chip 1 are not displaced in the moving direction of the printing paper P while the ink landing position of the (N+1)-th printer-head chip 1 is displaced in the moving direction of the printing paper P upward on the plan view.
In such a manner, in the case where the ink landing position is displaced in the moving direction of the printing paper P between the printer-head chips 1, the displacement appears stepwise as shown on the plan view. However, this landing position displacement appears as a step only at a printing start position or completion position, and it is not so conspicuous as the above-mentioned displacement in the lining-up direction of nozzles. Therefore, this displacement scarcely affects the image printing quality.
In addition, in the case where the ink landing position is displaced as described above, the conspicuousness of the stripe depends on the kind of images to be printed. For example, in documents, since white spaces are great, even when a stripe is produced, it is inconspicuous. Whereas when photographic images are printed with full color on the substantially entire region of the printing paper P, even a small white stripe may be conspicuous.
In the above-description, the ink landing displacements in the lining-up direction of nozzles and in the moving direction of the printing paper P are illustrated; in practice, pitch errors between the juxtaposed printer-head chips and displacements in the rotational direction may also be produced.
(Second Problem)
In a printer-head chip 1 with each ink liquid chamber having one heating resistor, ink priming (film boiling) by the heating resistor is performed only at one time. However, as shown in FIG. 13, in the case where each ink liquid chamber 2 has the heating resistor 3 divided into two, a difference is produced in the time until arriving at the temperature at which each heating resistor 3 film-boils ink (time until bubbles are produced), so that there may be a problem that the two heating resistors 3 may not film-boil the ink simultaneously.
In such a manner, if there is a difference in the time until arriving at the temperature at which two heating resistors 3 film-boil ink, the ink ejection angle deviates from the vertical direction, so that there is a problem of reduction in printing image quality due to the displacement in the ink landing position, as described above.
FIG. 17 includes graphs showing a relationship between a time difference until ink bubbles are produced by each heating resistor and the ink ejection angle, when divided heating resistors shown in FIG. 13 are provided. The values in these graphs are obtained by computer simulation. In these graphs, the X-direction (remark: not referred to the abscissa of the graph) is a lining-up direction of nozzles (the juxtaposing direction of the heating resistors), while the Y-direction (remark: not referred to the ordinate of the graph) is a direction perpendicular to the X-direction (the conveying direction of the printing paper).
In addition, the difference in time until bubbles are produced is plotted in the abscissa as data in these graphs; in examples shown in FIG. 17, a time difference of 0.04 μsec is equivalent to a resistance difference of 3%, and a time difference of about 0.08 μsec is equivalent to a resistance difference of about 6%.
As is understood from these graphs, the displacement of the ink ejection angle in the X-direction increases as the difference in time until bubbles are produced increases, while the displacement of the ink ejection angle in the Y-direction is scarcely affected by the difference in time until bubbles are produced.
FIGS. 18 and 19 are graphs showing actual measurements obtained from an actually manufactured printer-head chip with each ink liquid chamber having the heating resistor divided into two, as shown in FIG. 13. This printer-head chip has 336 nozzles, and the displacements of the ink landing position were measured for each nozzle in the X-direction (the lining-up direction of the nozzles and the juxtaposing direction of the heating resistors) and the Y-direction (the direction perpendicular to the X-direction). FIG. 19 shows displacements by plotting the displacement of the ink landing position in the X-direction in the abscissa and the displacement of the ink landing position in the Y-direction in the ordinate.
As is understood from these graphs, in the printer-head chip having the heating resistor divided into two, the ink landing position is displaced in the X-direction rather than in the Y-direction.
In FIG. 13, the range of the ink ejecting displacement for each nozzle is expressed by a phantom line. If the ink landing position is displaced in the X-direction, the ink landing displacement range forms an ellipse longitudinally extended in the lining-up direction of the nozzles. If a plurality of such printer-head chips are arranged to form a line head, white stripes or stripes may be liable to be produced, as described above.